In the field of fast inspecting a vehicle based on radiation scan, the current trend is to inspect a subject vehicle while it is traveling, since it can significantly improve the efficiency of security inspection. However, due to the reason that radiation rays of high energy is harmful to human body, during inspection of the subject vehicle, the cab (i.e. the head portion of the vehicle) should be blocked from the rays, to prevent the high-energy radiation rays from being emitted to the driver.
Presently, a most commonly used method is detecting the position of the vehicle by a plurality of photoelectric switches or light curtains as well as ground sensor coils installed in the inspection channel, and when the cab has passed a beam emitting region of the radiation rays (i.e. scanning region of the radiation rays), the radiation-ray source is controlled to emit a beam to scan the compartment behind the cab. However, errors are often generated in determination by means of optical switches or light curtains, which imposes a high risk to the safety of the driver.